The invention relates to an optical transmission system comprising a laser diode and, coupled to the laser diode, a photodiode whose photocurrent is applied to a control circuit for adjusting the optical power of the laser diode, which control circuit generates a first and a second setting parameter for respectively setting a modulation current and a bias current, and comprising an arrangement for determining a differential quotient of the optical power and the laser current flowing through the laser diode. The invention particularly relates to an optical transmitter for such a transmission system.
Such optical transmitters are used, for example, in cable digital transmission systems in which the transmission of information signals is effected via light waveguides.
From German Patent Application DE 38 17 836 A1 laid open to public inspection is known an optical transmitter comprising a laser diode, which transmitter comprises a control circuit for adjusting the optical power of the laser diode. Adjustment of the optical power is necessary because the characteristic line of the laser diode relating optical power to current, changes with ageing processes. In the event of an aged laser diode the dip in the characteristic line is shifted towards higher laser diode currents, and the slope (or "steepness") of the line is reduced especially for laser diode currents in the region above the dip. The laser diode current at the dip of the characteristic line is referenced the threshold current. In said state of the art publication a photodiode is used as a monitor diode which converts a pulse-shaped laser diode optical signal, on which a pulse-shaped pilot signal is superimposed, into a photocurrent. This photocurrent is applied to the control circuit which comprises two analog adjusting branches, one for maintaining the average optical power and the optical power in a state of rest at a constant level, and the other for adjusting a bias current consisting of only a D.C. portion of the laser current above the threshold current level. The laser diode generates the rest-state optical power when it is triggered only by the bias current. The threshold current must not be greater than the bias current, so that the adjusting region of the laser diode will be situated above the dip of its characteristic line. Distortions of the light signal of the laser diode are thereby avoided. The control circuit is based on a linearized model of the characteristic line of the laser diode. The ageing process of the laser diode may, however, cause an overproportional flattening of the characteristic line of the laser diode.